1. Field of the Invention
The present invention relates to a power control circuit for use in radio transmitters such as portable telephone sets.
2. Description of the Prior Art
In movable telephone systems such as a cellular phone system, it is the base station that controls or changes the transmission output of each of the mobile stations configured. This scheme is adopted so that the distance of radio transmissions stemming from each mobile station may be minimized and that the same frequency may be used by different mobile stations in different locations without interference therebetween. This allows the system to accommodate, under its control, the largest possible number of mobile stations that may communicate with one another.
To have its transmission output changed externally, the mobile station employs a transmission output section shown illustratively in FIG. 4. In FIG. 4, reference numeral 11 is a microphone; 12 is a transmission circuit; 13 is a power amplifier; 14 is a microstrip line; 15 is a band-pass filter for an outgoing channel; and 16 is a transmitter-receiver antenna.
During a transmission, a voice signal S11 from the microphone 11 is fed to the transmission circuit 12. The transmission circuit 12 converts the voice signal S11 to an outgoing channel transmission signal (FM signal) S12. The transmission signal S12 is amplified in power by the power amplifier 13 to become a signal S13. The power-amplified signal S13 passes through the microstrip line 14 and a filter 15 before reaching the antenna 16. From the antenna 16, the signal is transmitted by radio to the base station.
Reference numeral 20 represents a microcomputer for system control. The microcomputer 20 receives from the base station predetermined command signals or data via the transmission circuit 12 and via a reception circuit, not shown. In turn, these command signals or data allow the microcomputer 20 to control the transmission circuit 12 and the reception circuit.
Reference numeral 30 represents an APC circuit that controls the transmission output. In operation, the microcomputer 20 loads into a latch circuit 31 data D31 for designating the transmission output. From the latch circuit 31, the data D31 are sent to a D/A converter 32 for conversion to an analog voltage V32. The voltage V32 is supplied to a voltage comparison circuit 33 as the reference voltage. The signal S13 from the power amplifier 13 is sent to a detection circuit 34. This causes a DC voltage V34 to be output, the level of the voltage V34 corresponding to the level (i.e., amplitude ) of the transmission signal S13. The DC voltage V34 is fed to the comparison circuit 33 for comparison with the reference voltage.
The emitter and collector of a transistor 36 are connected in series between a power terminal 35 and the power line of the amplifier 13. A comparison output S33 from the voltage comparison circuit 33 is sent via a drive circuit 37 to the base of the transistor 36 as its control signal.
Thus the operating voltage fed to the amplifier 13 via the transistor 36 is varied in accordance with the comparison output S33. As the operating voltage of the amplifier 13 changes, so does the level of the transmission signal S13 output by the amplifier 13. The comparison circuit 33 provides feedback control such that V34=V32. That is, the level (=V34) of the transmission signal S13 from the amplifier 13 equals the level of the voltage V32.
The microcomputer 20 controls the value of the data D31 to manipulate the level of the transmission signal S13. That means the base station is capable of controlling the transmission output of each mobile station.
Generally, diodes have a temperature characteristic of about 2 mV/.degree. C. In the setup of FIG. 4, the transmission signal S13 from the amplifier 13 is detected by a diode 34Da in the detection circuit 34 in order to obtain the voltage V34 indicating the level of the signal S13. It follows that the level of the voltage V34 varies with temperature.
If the mobile station is a mobile phone set mounted on board a vehicle, the phone body containing the transmission section shown in FIG. 4 is located in the vehicle's trunk. In such an environment, the temperature change to which the diode 34Da is subjected is significant. That means there occur appreciable temperature-induced changes in the detected output voltage V34. As a result, the transmission output fluctuates considerably.
The adverse effect above is countered conventionally by raising the level of the transmission signal S13 fed to the detection circuit 34. This increases the detected output voltage V34, which in turn reduces proportionately the temperature change in the voltage V34 and thus lowers the fluctuation of the transmission output. However, having the amplifier 13 feed the transmission signal S13 to the detection circuit 34 constitutes a loss of power from the viewpoint of transmitting the signal S13 via the antenna 16 in practice. The higher the raised level of the transmission signal S13 fed to the detection circuit 34, the greater the loss of power.
One way to bypass the disadvantage above is proposed in U.S. Pat. No. 4,523,155. That patent discloses a circuit arrangement wherein a diode 34Db constitutes a temperature compensation circuit. The temperature-induced change in the terminal voltage of the diode 34Db is arranged to cancel out the temperature-induced change in the detected output voltage V34 of the diode 34Da.
The proposed temperature compensation scheme has a number of disadvantages. For one thing, as shown in FIG. 4, the scheme requires the detecting diode 34Da and the temperature compensating diode 34Db to be kept at the same temperature. That is, the two diodes 34Da and 34Db must be located close to each other. Another disadvantage is as follows: the two diodes 34Da and 34Db allow different bias currents to flow therethrough. As a result, the forward voltage drops of the two diodes differ from each other, causing an offset voltage to be included in the detected output voltage V34 of the detection circuit 34.